Windows builds for macro targets in release mode break the compiler

[broken-bytes]

Description

Issue

When building any macro target on Windows in release mode, the Swift Compiler gets hung.
The last output with -vv set is always -o F:\Projects\MacroTest\.build\x86_64-unknown-windows-msvc\release\SwiftSyntax.build\SyntaxTypeNodes.swift.o.

Test setup

• Any recent Swift version, tested on 5.9.2-release and 5.11 development.
• Default macro package(In fact any macro package)
• Recent swift-syntax dependency, tested with 509.0.0 and 509.1.1

Steps to Reproduce

• swift package init --type=macro
• swift build -c release -vv
• Compiler gets hung

[ahoppen]

Tracked in Apple’s issue tracker as rdar://122442648

[broken-bytes]

Is this something that was already known? I can't seem to see the issue via the link

[hassila]

Is this something that was already known? I can't seem to see the issue via the link

You're not supposed to - that's Apples internal bug tracking system, so presumably they filed (or had) an internal issue covering the same problem.

[ahoppen]

@compnerd Have you seen this before?

[compnerd]

I have not - might be interesting to get this under a debugger.

[broken-bytes]

@compnerd If I can assist in any way feel free to instruct me. If you need additional logs or anything.

[compnerd]

What is really interesting is that we definitely would be building this file in release mode all the time - swift-syntax is part of the toolchain build and so we have this continuously building on Swift CI, GHA for The Browser Company's builds of the toolchain, and multiple developers building locally.

[broken-bytes]

Does building a macro package work in release mode for you locally? From what you said it sounds like?

[ahoppen]

Could the difference be that you are building using CMake, @compnerd while @broken-bytes is building using SwiftPM?

[compnerd]

@ahoppen possible - that is a difference between the builds, though feels surprising.

[broken-bytes]

Not too surprising to be honest. I had a couple of issues with SPM whereas the same code with CMake worked. Cxx interop, for example.

[broken-bytes]

FYI: Just tested, this is still broken in the same way on Windows a year later with Swift 6.1.2.
In fact it is even the very same file:

H:\Projects\Playground\scripting\engine> swift build -c release 06/06/25 21:37:52
Building for production...
[23/28] Compiling SwiftSyntax601 Empty.swift

This runs forever.

[broken-bytes]

Another FYI: As @compnerd said before they were building with CMake. Can confirm that CMake works fine on the very same machine and macros are built just fine in release mode.